An alkaline secondary battery, which uses a metal compound, for example, nickel hydroxide, for a positive electrode and a hydrogen-absorbing alloy for a negative electrode, such as a nickel-metal hydride battery, has lately been attracting attention as a high energy density battery for replacing nickel-cadmium batteries. The reason for this attention is that nickel-metal hydride batteries have more than twice the capacity of nickel-cadmium batteries, and have better environmental suitability.
The hydrogen-absorbing alloys used for a negative electrode of nickel-metal hydride batteries include Mm series hydrogen-absorbing alloys, where "Mm" is a so-called "Misch Metal" and is a mixture of rare earth elements, such as La, Ce, Nd, Pr, etc., and Laves phase alloys.
It sometimes happens that the surface of a hydrogen-absorbing alloy is coated with oxide formed by natural oxidization. This coating inhibits the absorption of hydrogen gas. When a battery having a hydrogen-absorbing alloy electrode is manufactured using an oxidized hydrogen-absorbing alloy, the battery cannot provide sufficient discharge capacity. It is also a problem that internal pressure of the battery increases during charge.
A method to immerse a hydrogen-absorbing alloy in an acid solution such as hydrochloric acid and to dissolve an oxide coating on the surface of the alloy to remove the coating from the alloy surface and expose electrochemically active portions of the surface of the alloy is disclosed in Japanese Patent Laid-Open Publication No. 5-225975. The method is somewhat useful to increase electrochemical activity of a hydrogen-absorbing alloy electrode.
However, even if electrochemically active portions are exposed on the surface of the alloy, they are easily oxidized in air. The conventional method to treat a hydrogen-absorbing alloy is immersing and stirring alloy particles in an acid solution to remove the oxidized coating of the surface and to increase the electrochemical activity of a hydrogen-absorbing alloy electrode. However, since not only the oxidized surface but also the hydrogen-absorbing alloy itself is corroded by the acidic solution, the surface of the alloy is easily oxidized.
There still has not been obtained a solution to increase discharge capacity of an alkaline secondary battery having a hydrogen-absorbing alloy electrode and to inhibit an increase of internal pressure of the battery during discharge.